Stationary-vehicle structure from motion

ABSTRACT

A vehicular structure from motion (SfM) system can store a number of image frames acquired from a vehicle-mounted camera in a frame stack according to a frame stack update logic. The SfM system can detect feature points, generate flow tracks, and compute depth values based on the image frames, the depth values to aid control of the vehicle. The frame stack update logic can select a frame to discard from the stack when a new frame is added to the stack, and can be changed from a first in, first out (FIFO) logic to last in, first out (LIFO) logic upon a determination that the vehicle is stationary. An optical flow tracks logic can also be modified based on the determination. The determination can be made based on a dual threshold comparison to insure robust SfM system performance.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/121,012, filed Sep. 4, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/235,516, now U.S. Pat. No. 10,108,864, filed onAug. 12, 2016, which claims priority to Indian provisional patentapplication No. 7027/CHE/2015, filed in the Indian Patent Office on Dec.29, 2015, each of which is hereby incorporated by reference herein inits entirety.

TECHNICAL FIELD

This disclosure relates to computer vision systems and methods. Moreparticularly, this disclosure relates to systems and methods forproviding structure-perceptive vision to vehicles for autonomous ordriver-assisted navigation. This disclosure particularly relates tostationary-vehicle structure from motion.

BACKGROUND

Structure from motion (SfM) is a range imaging technique for estimatingthree-dimensional (3D) structures from two-dimensional (2D) imagesequences from a single camera. Because it can recover 3D informationfrom a single, inexpensive camera, it can be a cost-effective solutionas compared to stereo imaging systems or range sensors like lidar orautomotive radar. SfM can also increase the robustness of advanceddriver assistance systems (ADAS) while working in tandem with othersensors, such as radar, to provide automatic emergency braking (AEB).

However, when a camera used as part of a structure-from-motion system isstationary, the captured 2D image sequences can appear the same, andthus may fail to provide information regarding 3D structure. Under suchconditions, SfM fails to recover 3D range information, generallydescribed by of a set of points in 3D space, from the 2D imagesequences. Consequently, in automotive scenario having anoutward-looking camera placed inside the vehicle, SfM may not be usefulwhen the vehicle is not moving.

Prior approaches for obtaining SfM in stationary scenarios carry forwardthe point cloud generated when the camera was moving. Such approachessometimes account for objects moving into and out of a stationary-camerascene by applying background subtraction or other segmentationtechniques to preserve the 3D points pertaining to the background.Although such techniques may be able to handle cases involving theremoval of 3D points where a new object has come in the scene, they maynot be able to handle cases requiring the addition of 3D points inregions of the scene where objects have moved out of the scene.

SUMMARY

This disclosure relates to systems and methods for obtaining structurefrom motion (SfM) in stationary vehicles. The disclosed systems andmethods use a novel technique to provide 3D information even when thecamera is not moving.

In an example, a vehicular structure from motion (SfM) system caninclude an input to receive a sequence of image frames acquired from acamera on a vehicle, a memory to store a finite number of the frames ina frame stack according to a frame stack update logic, and one or moreprocessors to implement the frame stack update logic, detect featurepoints, generate optical flow tracks, and compute depth values based onthe image frames, the depth values to aid control of the vehicle. Theframe stack update logic can select a frame to discard from the stackwhen a new frame is added to the stack. The frame stack update logic canbe changed from a first in, first out (FIFO) logic to last in, first out(LIFO) logic upon a determination that the vehicle is stationary.Similarly, the frame stack update logic can be changed from the LIFOlogic back to the FIFO logic upon a determination that the vehicle ismoving.

One or more processors can implement an optical flow tracks logic toprune optical flow tracks generated from corresponding feature points indifferent frames. Upon a determination that the vehicle is stationary,the optical flow tracks logic can be changed from pruning based on thelast-computed set of tracks to pruning based on the last set of trackscomputed from a frame acquired while the vehicle was moving. Similarly,upon a determination that the vehicle is moving again, the optical flowtracks logic can be changed from pruning based on the last set of trackscomputed from a frame acquired while the vehicle was moving to pruningbased on the last-computed set of tracks. By “last-computed set oftracks,” what is meant is the set of tracks computed from feature pointsin a plurality of frames immediately prior to the most recently computedset of tracks being pruned.

The determination that the vehicle is stationary can be made by the oneor more processors or by another component, which can make thedetermination by estimating the pose of the camera and computing atranslation vector relating the camera pose to a reference position, andthen making two comparisons.

In the first comparison, a first value can be compared with a firstthreshold, the first value being the magnitude of the difference betweenthe translation vectors corresponding to acquired image frames that areconsecutive in time. In the second comparison, a second value can becompared with a second threshold, the second value being the magnitudeof the difference between the translation vector corresponding to themost recently acquired frame and the translation vector corresponding tothe last frame acquired while the vehicle was moving.

The vehicle can then be determined to be stationary based on both thefirst and second comparisons. For example, the vehicle can be consideredto be stationary when at least one of the following conditions is met:the first value is less than the first threshold, or the second value isless than the second threshold. If neither condition is met, the vehiclecan be determined to be moving.

In another example, a method for SfM-based control of a vehicle canbegin with acquiring, from a camera on a vehicle, a sequence of imageframes in a frame stack having an update scheme. Then, the vehicle canbe determined to be stationary or moving. If the vehicle is determinedto be stationary, the frame stack update scheme can be modified from afirst in, first out (FIFO) scheme to a last in, first out (LIFO) scheme.Also, an optical flow tracks logic can be modified as described above.If, on the other hand, the vehicle is determined to be moving, the framestack update scheme can be modified from a LIFO scheme to a FIFO scheme.Also, the optical flow tracks logic can be reverted in behavior asdescribed above.

Another example method can begin with acquiring a sequence of imageframes from a camera on a vehicle, and continue by estimating the camerapose and computing a translation vector relating the camera pose to areference position. Then, the two threshold comparisons described abovecan be made, the vehicle can be determined to be moving or stationarybased on the comparisons, and the behavior of a structure-from-motion(SfM) depth determination system can be modified based on thedetermination. Depths can be determined based on the sequence of imageframes using the SfM depth determination system, and the vehicle can becontrolled based on the determined depths.

The SfM depth determination system can include a frame stack updatelogic, and modifying the behavior of the SfM depth determination systemcan include modifying the frame stack update logic from a FIFO behaviorto a LIFO behavior upon a determination that the vehicle is stationary,or vice versa upon a determination that the vehicle is moving.

The SfM depth determination system can also include an optical flowtracks logic to prune optical flow tracks generated from correspondingfeature points in different frames. Modifying the behavior of the SfMdepth determination system can include modifying the optical flow trackslogic from pruning based on the last-computed set of tracks to pruningbased on the last set of tracks computed from a frame acquired while thevehicle was moving, upon a determination that the vehicle is stationary,or vice versa upon a determination that the vehicle is moving.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example structure from motion (SfM) system used forvehicular control.

FIGS. 2A and 2B show example SfM systems.

FIG. 3 is a diagram illustrating triangulation for SfM.

FIGS. 4A and 4B respectively show representative point cloud output fromthe example SfM systems.

FIGS. 5A-5E illustrate the updating of a frame stack by an example framestack logic.

FIGS. 6A-6G illustrate the updating of a frame stack by an exampleadaptive frame stack logic.

FIGS. 7A-7J illustrate how optical flow is updated by an example opticalflow tracks logic.

FIGS. 8A-8J illustrate how optical flow is updated by an exampleadaptive optical flow tracks logic.

FIG. 9 is a flow chart showing an example method of updating a framestack.

FIG. 10 is a flow chart showing an example method of updating flowtracks.

FIG. 11 is a flow chart showing an example method of performing astationary check.

FIG. 12 is a flow chart showing an example method of updating a framestack.

FIG. 13 is a flow chart showing an example method of updating flowtracks.

FIG. 14 is a flow chart showing an example method of performing astationary check.

DETAILED DESCRIPTION

Systems and methods are described for determining three-dimensional (3D)structures from sequences of two-dimensional (2D) images acquired from astationary vehicle. The systems and methods of the current disclosurecan provide a dense 3D reconstruction of a scene, even when the vehicleis not moving (i.e., when an onboard camera is stationary). Such acapability is useful in an automotive scenario, as when an automobile iswaiting at a traffic light or stop sign and accurate depth informationindicative of a crossing pedestrian is needed to determine the locationof the pedestrian and prevent the automobile from accelerating into thepedestrian. The systems and methods of the present disclosure canprovide the depth information necessary to make such a determination,thus enhancing the navigational capabilities of the vehicle.

As the name “structure from motion” implies, SfM techniques generallyare reliant on a moving camera to compute the desired 3D informationregarding the surrounding scene from the 2D sequence of images generatedby the camera. In the context of vehicular SfM systems, the camera canbe mounted in a location in the vehicle, such as behind a windshield orother part of the vehicle. Once the vehicle starts moving, the SfMsystem is fed by a sequence of 2D images from the onboard camerarepresentative of the captured scene, and can generate sensible 3Dreconstructions.

In an example system like that shown in FIG. 1, camera 3 can be mountedto vehicle 1 to deliver image frames to SfM system 10. Vehicle 1 isillustrated as an automobile but can be any kind of vehicle, includingwatercraft, aircraft, or other types of land vehicles. Camera 3 can bemounted anywhere on the vehicle, including behind a windshield, orwithin the front, rear, or side body. SfM system 10 may also be fedinformation about the travel of vehicle 1 by inertial measurement unit(IMU) 5, which may comprise, for example, a 3D accelerometer andgyroscope and/or a speedometer.

SfM system 10 generates depth information about the surrounding scene,which may be, for example, in the form of 3D point clouds indicative ofdistances to obstacles, hazards, and/or targets. SfM system 10 deliverssuch information to vehicle controller 50, which uses the depthinformation to activate or deactivate vehicle control systems that caninclude propulsion systems, braking systems, steering or maneuveringsystems, safety or restraint systems (e.g., seat belts, airbags, poweredwindows, and door locks), signaling systems (e.g., turn signals, blinkerlights, horns, and sirens), and communication systems. Vehiclecontroller 50 may also be fed information from other sensor systems suchas radar- or lidar-based detection systems and/or from manual pilotingcontrols.

FIG. 2A shows an example process flow for SfM system 10. A cameragenerates a sequence of 2D frames beginning with frame 1 and ending withnewest frame t 12. Only two frames, t 12 and preceding sampled frame t−114, are illustrated in FIG. 1, but more than two frames may beprocessed. A subset of the frames is processed by the SfM system 10 toderive 3D information about the surrounding scene.

Vision processor 16 can detect interest points using interest pointdetector 18. Interest point detection may also be called feature pointdetection. Vision processor 16 can further perform sparse optical flowcalculation 20. The vision processor 16 may be, for example, an EmbeddedVision/Vector Engine (EVE), which is a specialized, fully-programmableprocessor with pipelines and units to accelerate computer visionalgorithms, having a memory architecture better suited for sustainedinternal memory bandwidth for compute intensive algorithms thangeneral-purpose processors. The vision processor 16 may also be ageneral-purpose processor, or the functions of the vision processor 16may be performed instead by a digital signal processor (DSP), such asDSP 22.

Interest point detection 18 processes an individual image frame to findfeatures, such as corners and/or edges that can yield points betweendifferent 2D images that consistently correspond to the same 3D point inspace. Interest point detection 18 can use, for example, Shi-Tomashi orHarris methods to extract interest points, also called feature points.Given the detected feature points, sparse optical flow calculation 20calculates interframe velocity vectors for some feature points, as atdetected corners, for example. Optical flow calculation 20 can provideinformation describing how detected feature points have moved from oneimage frame to the next in the 2D image scene.

The several processes of fundamental/essential matrix computation 24,camera pose estimation 26, and 3D triangulation 28 can be performed bydigital signal processor (DSP) 22. Fundamental/essential matrixcomputation 24 can be used to prune inaccurate optical flow tracks offeature points delivered by vision processor 16. Camera pose estimationcan be computed based on the image data from frames 12 and 14, can bedetermined from an external sensor value, such as can be provided frominertial measurement unit (IMU) 5, or can be determined from somecombination of these methods. 3D triangulation 28 provides 3D sparsepoints 30 representative of distances to objects in the surroundingscene, which can be output to other systems, such as vehicle controller50 in FIG. 1 to control a vehicle to accelerate, brake, turn, swerve,etc to perform vehicle functions such as to approach targets, avoidobstacles and hazards, maintain or change lanes, etc.

An example of triangulation is shown in FIG. 3. Three successiveselected frames n 302, n+1 304, and n+2 306 contain respective keypoints 308, 310, and 312 that are identified as likely corresponding tothe same point in 3D space, which is the 3D point 314 beingreconstructed. The identification can be performed, for example, by akey point detection and optical flow methodology. Individual tracks 320,322 define the motion of the key points 308, 310, and 312 from frame toframe. The position of the 3D point 314 in the 3D scene can bedetermined by computing the back-projected 3D ray 316 originating fromthe camera center of frame n 302 and passing through key point 308 inframe n 302, computing the back-projected 3D ray 318 originating fromthe camera center of frame n+2 306 and passing through key point 312 inframe n+2 306, and finding the intersection 314 of the two rays 316,318.

Only a subset of captured frames need be used to compute 3D information,since the processing of each frame incurs a computational cost, andreal-time processing is desirable, necessitating the fastest possiblecomputation and thus the lowest computational cost. The subset of framesmay consist only of a number of the most recent frames, since olderframes are generally less informative about the current state of thescene. The frames from which the subset is drawn may be temporallysampled periodically from all frames acquired by the camera, but thetemporal sample rate may be less than the native frame rate of thecamera. Thus, for example, the camera may be capable of acquiring 100frames per second, but in some examples it may be that only 30 framesper second are used for obtaining SfM. In other examples, it may be thatonly 10 frames per second are used. The exact frame rate used may dependupon the application and/or the speeds involved. For example, duringfreeway driving, when the vehicle is moving at fast speeds, 30 framesper second may be used for obtaining SfM, whereas during a slow-movingpark-assist application, it may be that only 10 frames per second areused. The subset of frames to be processed to obtain SfM, e.g., uponwhich triangulation can be performed to arrive at a 3D point cloud,together make up the frame stack. Frames can be removed from the framestack as new frames are added to the frame stack.

Thus, for example, while frames 1 through t−1 may initially be used tocompute a 3D point cloud, when a new frame t is delivered, frame 1 maybe discarded from the frame stack and thus from consideration incomputing 3D information. As such, the new frame stack may consist offrames 2 to t−1 and frame t. A 3D point cloud can be generated usingthis new frame stack. This first in, first out (FIFO) flow of frames cancontinue as new frames are captured. Captured frames can be stored in apool of frames 32 which can be made available to 3D triangulation 28.Pool of frames 32 can be stored, for example, in a non-transitorycomputer-readable memory. Computer-readable instructions for carryingout the different steps described can similarly be stored on the same ora different memory.

In an automotive scenario, a host vehicle may come to a halt, as at atraffic light or intersection, or in slow-moving traffic. The vehiclemay later start moving again. At various points during such stop-gosituations, the onboard camera ceases to be in motion, and the absenceof camera motion can result in a loss of accurately computed depthinformation and a faulty SfM reconstruction.

When the camera is stationary, all the frames in the frame stack may allcontain similar 2D information about the static scene, and there willnot be a sufficient baseline for triangulation to compute a 3D pointcloud. Such failure can be characterized by a paucity of reconstructedpoints, by points having depth values (i.e., estimated distances fromthe camera) that are inaccurate as compared to corresponding real scenedepths, and/or by temporal instability in the set of feature pointstracked.

The result of the failed reconstruction performed by SfM system 10 inthe seconds immediately succeeding a vehicle stop is as shown in FIG.4A. In the depicted stationary-camera video frame, overlain 3D pointsdetected as nearer are illustrated with crosses 402 while pointsdetected as further away are illustrated with X's 404. It can beobserved in FIG. 4A that there are relatively few points and also theyare wrongly reconstructed as all of them indicate low distance, eventhose points found on distant trees. Estimated distances in units ofmeters superimposed on the image of FIG. 4A also show, for example, thatthe back of a truck 406 two vehicles in front of the camera-equippedvehicle is detected as being nearer than the back of a car immediatelyin front of the camera-equipped vehicle. The more distant truck 406 isthought to be 3.2 meters away whereas the nearer car is thought to be3.8 meters away. This illogical inaccuracy is apparent to a humanobserver and illustrates the failure of the SfM system 10 in FIG. 2A.

FIG. 2B illustrates SfM system 11 that alters the SfM system 10 of FIG.2A any one or more of at least three ways. Frame stack logic 202 canintelligently, adaptively switch between various frame stack updatemethods depending on whether the camera and vehicle are in motion ornot. Optical flow tracks logic 204 can alter how optical flow is updatedand pruned. Stationary check 206 can alter how the SfM system 11determines whether the camera is coming to a halt or not.

The stationary-camera scenario alteration of the frame stack logic 202in SfM system 11 can provide sufficient baseline for triangulation toaccurately compute the 3D point cloud 30 and also handle changes in thescene itself, such as objects moving out of the scene or entering thescene.

FIGS. 5A-5E illustrate the FIFO-updated frame stack update logic of SfMsystem 10. Frame numbers prefixed with the letter M represent framescaptured while the vehicle is moving, while frame numbers prefixed withthe letter S represent frames captured while the vehicle is stationary.In the illustrated examples of both FIGS. 5 and 6, the sequence offrames proceeds as M0, M1, M2, M3, M4, M5, M6, M7, S8, S9, S10, S11,S12, S13, S14, M15, M16, M17, and so on, indicative of a brief stopfollowed by a resumption of movement. The number of frames in the framestack 502 can be determined by a sliding window. In the illustratedexample, the sliding widow size used for triangulation is 6 frames.

Thus, in FIG. 5B, as compared to FIG. 5A, frame M0 504 is replaced by M6506 after the first set of frames are processed, then, as shown in FIG.5C, M1 508 is replaced by M7 510, and so on. When static frame S8 512 iscaptured and processed, as illustrated in FIG. 5D, M7 510 and S8 512 areused for matching and triangulation. However, because there is no motionbetween these M7 510 and S8 512, there is no sufficient baseline betweenthese two frames, and triangulation will fail.

By contrast, SfM system 11 can handle the frame stock logic differentlywhen the vehicle, and hence the camera, is not moving. Rather than usinga FIFO frame stack logic, frame stack logic 202 can use a last in, firstout (LIFO) flow of the frames. In general terms, instead of removingframe 1 when new frame t comes in, new frame t can replace frame t−1,such that the new frame stack would consist of frames 1, 2, . . . t−2,and t. This would ensure that there is sufficient baseline fortriangulation to succeed. Also, since the point cloud data is recomputedfor every new frame, it will be able to handle new information in scene,such as objects moving in-out of the scene. An example of the LIFO framestack logic 202 of SfM system 11 is illustrated in the following tableand in FIGS. 6A-6G.

Frames in frame stack (sliding window state) Frame removed M0 M1 M2 M3M4 M5 M1 M2 M3 M4 M5 M6 M0 M2 M3 M4 M5 M6 M7 M1 M2 M3 M4 M5 M6 S8 M7 M2M3 M4 M5 M6 S9 S8 M2 M3 M4 M5 M6 S10 S9 . . . M3 M4 M5 M6 S14 M15 M2 M4M5 M6 S14 M15 M16 M3 M5 M6 S14 M15 M16 M17 M4 M6 S14 M15 M16 M17 . . .M5

FIGS. 6A-6C exhibit the same FIFO frame stack logic as in FIGS. 5A-5C.During this period the vehicle and camera are moving, so, for example,in FIG. 6A, frames M0 504 through M5 514 are considered for processingand the 3D point cloud 30 is generated by SfM system 11 using those sixframes. When a new frame M6 506 is captured, the frame stack is updatedin normal FIFO manner with M0 504 being replaced by M6 506. Frames M1508 through M6 506 are considered for processing by SfM system 11, andthe 3D points 30 are generated. This process is continued until astationary frame S8 512 is encountered. When frame S8 512 is captured,instead of replacing M2 516 with S8 512 as in FIG. 5D, frame stack logic202 replaces frame M7 510 with frame S8 512. Since frame S8 512 has nocamera motion compared to M7 510, M7 510 is discarded. This ensures thatSfM system 11 has sufficient baseline for triangulation to succeed byconsidering frames M2 516 through M6 506 and S8 512.

Similarly, when the next stationary frame S9 518 is captured, as shownin FIG. 6E, frame S8 512 is replaced with frame S9 518, and so on, in aLIFO manner. FIG. 6F depicts the replacement of frame S9 518 with laststationary frame S14 520 before the vehicle and camera begin movingagain. When a moving frame M15 522 is encountered, as shown in FIG. 6G,the frame stack logic 202 resumes replacement of frames in frame stack602 in a FIFO manner by replacing M2 516 with M15 522, and so on, aslong as the vehicle remains in motion.

An example output of SfM system 11 using frame stack logic 202 is shownin the video frame of FIG. 4B. As can be observed by comparing FIG. 4Bto the video frame of FIG. 4A, in which the frame stack was generatedusing only FIFO logic, even in the situation where the camera-equippedvehicle has stopped, sensible 3D reconstruction is obtained. Asexamples, the number feature points identified is greatly improved,background trees are correctly identified as distant, and the back oftruck 406 is correctly estimated at a more distant 12 meters from thecamera as compared to the back of the immediately leading car which isestimated at being 2.8 or 2.9 meters distant from the camera. It mightalso be observed from motion video of the output represented by FIG. 4Bthat the feature points exhibit temporal stability, with largely thesame feature points being represented from frame to frame even while thevehicle is stationary.

SfM system 11 can also include optical flow tracks logic 204, which canuse a modification of a pyramidal Lucas-Kanade implementation of opticalflow to find feature matches between two frames, such as between frame n302 and frame n+2 306 shown in FIG. 3. The Lucas-Kanade method is adifferential method for optical flow estimation that assumes that theflow is essentially constant in a local neighborhood of each pixel underconsideration, and solves the basic optical flow equations for all thepixels in that neighborhood, by the least squares criterion.

Although tracks can be generated between any two frames, and the frameswhose feature points are compared need not be consecutive in thetemporal sequence of acquired frames, in order to reduce the number ofcombinations of frames to match, it may be that only consecutive framesin the frame stack are considered for matching. Thus, for example, inthe frames shown in FIG. 3, feature points in frame n 302 can be matchedwith corresponding feature points in frame n+1 304 to arrive at opticalflow tracks, and feature points in frame frame n+1 304 can be matchedwith corresponding feature points in frame n+2 306 to arrive at opticalflow tracks. Tracks that do not find a match in the last frame can beremoved and not considered for future frames. This can be done in orderto retain only the most reliable tracks in a given frame fortriangulation.

The disclosed improvement in the method of feature track generation inSfM system 11 is illustrated via the contrast between, on the one hand,FIGS. 7A-7J, which together make up one sequence, and, on the otherhand, FIGS. 8A-8J, which together make up a similar sequence that usesthe improved optical flow tracks logic 204. Both of the illustratedexamples take into account the adaptive frame stack logic 202. In theillustrated examples, frames acquired while the vehicle and camera aremoving are prefixed with the letter M, while frames acquired while thevehicle and camera are stationary are prefixed with the letter S. In theillustrated examples, the sliding window size used for triangulation is6, and the sequence of frames is M0, M1, M2, M3, M4, M5, M6, M7, S8, S9,M10, and so on.

Thus, as shown in FIG. 7A, optical flow tracks T_(R) 1 702 can becomputed between feature points in first two frames M0 504 and M1 508.Then, when new frame M2 516 comes in, optical flow tracks can becomputed between feature points in frames M2 516 and M1 508, as shown inFIG. 7B. These computed tracks are merged with the previously computedtracks T_(R) 1 702 to arrive at tracks T_(R) 2 704. This merging can bedone to preserve the “history” of the tracks computation. For example,there might be recognized feature points that are present in all threeframes M0 504, M1 508, M2 516, and since the latest step of optical flowtrack generation was done by performing matching of these points in M2516 with respect to M1 508 only, the merging of tracks T_(R) 1 702 willincorporate the prior computed information if the feature point matchfound in M1 508 also has a match in M0 504. In this way, T_(R) 2 704 is“pruned” based upon the feature point matches found between frames M2516 and M1 508 as well as T_(R) 1 702, which itself consists of featurepoint matches between frames M1 508.

Similarly, as shown in FIG. 7C, when new frame M3 524 is acquired,optical flow tracks can be computed between feature points in frames M3524 and M2 516, and these tracks can be pruned by looking at tracksT_(R) 2 704 to arrive at tracks T_(R) 3 706. This sequential method ofupdating tracks can be real-time and can be performed on embeddedsystems of limited computational power because the method involvesfeature point matching between only two frames and the number of tracksis limited to a reasonable number that can be handled in an embeddedsystem.

The method continues in the same manner in FIGS. 7D, 7E, 7F, and 7G. Asshown in FIG. 7E, frames M0 504 through M5 514 are considered for trackformation and all the tracks that find a match in M5 514 are retainedand considered for 3D point cloud generation. When the new frame M6 506is captured, as shown in FIG. 7F, frame MO 504 is replaced by frame M6506 in the frame stack, since, in the illustrated example, the slidingwindow size is 6. Lucas-Kanade optical flow can be done only for framepair M5 514 and M6 506 and the tracks that find a match in M6 506 can beretained for further processing.

FIG. 7H illustrates the condition when a static frame, S8 518, isacquired. As described previously with regard to adaptive frame stacklogic 202, the frame stack can be updated in a LIFO manner when thecamera is stationary. Thus, frame M7 510 can be removed from the stack,instead of removing frame M2 516 from the stack. The feature pointmatching can then be performed between frames M6 506 and S8 512.

Because T_(R) 7 714 pertains to matches found between M7 510 and M6 506,and because the adaptive frame stack logic 202 operates as though M7 510does not exist, it would be wrong to prune tracks found between S8 512and M6 506 using tracks T_(R) 7 714 to arrive at tracks T_(R) 8 716.Doing so would, for example, fail to account for the history of trackswhere matching feature points are found in M6 506 and S8 512 but missingfrom M7 510. As such, the above-described method of track formation, asillustrated in FIGS. 7A-7J, can produce undesirable results during astop-go camera motion scenario.

When frame S8 is 512 captured, as shown in FIG. 7H, frame M7 510 isreplaced by frame S8 512. However, tracks T_(R) 7 714 would be prunedsuch that tracks that had a match in frame M7 510 were retained. Usingthese tracks for finding a match with S8 512, as indicated in FIG. 7H,would yield an incorrect result, because tracks pertaining to M6 506need to be considered to find match in S8 512. One way to do so may beto perform Lucas-Kanade optical flow matching for all six frames M2 516through M6 506 and S8 512 again, but doing so would increasecomputational complexity significantly whenever a stationary situationwould occur.

Instead, a back-up/restore mechanism of tracks can be used. A back-up ofthe un-pruned tracks and pruned tracks can be taken. When the currentframe is not stationary, the pruned tracks are restored and updatedbased on the match found in the current frame. However, when astationary frame is encountered, the un-pruned tracks are restored andupdated based on the match found on the current frame. Only the prunedtracks are used for computing the 3D point cloud for any given set offrames.

This approach is explained with reference to FIGS. 8A-8J and the belowtable. The process in FIGS. 8A-8G appears identical to that of FIGS.7A-7G. As shown in FIG. 8F and in the second line of the table, tracksT_(R) 5 710 are used for pruning when new frame M6 506 is processed.Similarly, tracks T_(R) 6 712 are used when frame M7 510 is processed,as shown in FIG. 8G and in the third line of the table.

However, during stationary frames, instead of using pruned tracks, themethod can use the tracks that are not pruned. This can be done by abackup-restore mechanism, taking a back up of both pruned and unprunedtracks after the optical flow of each frame, then restoring theappropriate tracks based on a stationary check. Thus, for example, whenstationary frame S8 512 is processed, tracks T_(R) 6 712 are used andnot tracks T_(R) 7 714, as shown in FIG. 8H and in the fourth line ofthe table. Similarly, when frame S9 518 is processed, as illustrated inFIG. 8I, tracks T_(R) 6 712 are used to update the match and not tracksT_(R) 8 716.

When a moving frame is again encountered, such as frame M10 722 in FIG.8J, then tracks T_(R) 9 718 can be used for pruning, and not tracksT_(R) 6 712. For the purposes of illustration, the example in the belowtable has more stationary frames than illustrated in FIGS. 8A-8J. In thebelow table, the camera remains stationary through frame S14. Whenmoving frame M15 is encountered, tracks T_(R) 14 can be used, and nottracks T_(R) 6.

To summarize the below table, when moving frames M6 and M7 areencountered, then “tracks after pruning” T_(R) 5 and T_(R) 6 can berespectively restored and used for further processing. When stationaryframes S8 through S14 are encountered, “tracks before pruning” T_(R) 6can be restored and used for further processing. When moving frames M15and M16 are encountered again, “tracks after pruning” T_(R) 14 and T_(R)15 respectively can be used for further processing.

Tracks before Tracks after Sliding window state pruning pruning M0 M1 M2M3 M4 M5 T_(R)4 T_(R)5 M1 M2 M3 M4 M5 M6 T_(R)5 T_(R)6 M2 M3 M4 M5 M6 M7T_(R)6 T_(R)7 M2 M3 M4 M5 M6 S8 T_(R)6 T_(R)8 M2 M3 M4 M5 M6 S9 T_(R)6T_(R)9 M2 M3 M4 M5 M6 S10 T_(R)6 T_(R)10 . . . M3 M4 M5 M6 S14 M15T_(R)14 T_(R)15 M4 M5 M6 S14 M15 M16 T_(R)15 T_(R)16 M5 M6 S14 M15 M16M17 T_(R)16 T_(R)17 M6 S14 M15 M16 M17 . . . T_(R)17 . . .

Aside from the possible additions of frame stack logic 202 and opticalflow tracks logic 204, which together make up stationary vehicle flowbased processing, SfM system 11 can also include stationary check 206that can alter how the SfM system 11 determines whether the camera iscoming to a halt or not. The following process can be used in stationarycheck 206 to make a determination that a vehicle is stationary.

In an automotive scenario, typically, a vehicle and its associatedcamera comes to a halt gradually over time rather than abruptly.Consequently, a system that would wait for the vehicle and camera tocome to a stand-still position to take any action would do so too lateto recover an accurate 3D point cloud since most of the points would nothave sufficient baseline and hence would not be properly reconstructed.A threshold-based approach can be employed in order to determine if thevehicle and camera are stationary and the stationary vehicle flow basedprocessing can be enabled accordingly. Such processing can include theframe stack logic 202 and/or the optical flow tracks logic 204 asdescribed above.

The SfM 3D-point recovery process can rely on camera pose information,such as may be derived by camera pose estimation 12 in FIG. 2B. Asshown, the pose information can be computed from information provided byexternal sensors, such as inertial measurement unit (IMU) 5, frominformation in acquired images using, for example, a fundamental matrixdecomposition based technique or a perspective-n-point (PnP) technique,or by some combination of these. The pose information containsinformation describing the translation and rotation of the camera withrespect to a reference frame.

The translation information present in the pose matrix can be used todetermine if the camera is moving or not. A camera can be “considered”to be stationary even if it is not completely stationary; a camera thatis moving too slowly to recover accurate depth information from acquiredframes using conventional SfM techniques can be considered to bestationary. The camera is considered to be stationary if either of thebelow two conditions is true, or if both of them are true. First, if themagnitude of the difference between the translation vectors of capturedimage frames that are consecutive in time (i.e., Abs(T_(X)−T_(X−1))) isless than a threshold Th₁, the camera may be said to be stationary.Second, if magnitude of the difference between the translation vectorT_(X) of the current frame and the last moving frame in the frame stack(i.e., Abs(T_(X)−T_(last-moving))) is less than a threshold Th₂, thecamera may be said to be stationary.

If either of the above conditions is true, then the stationary vehicleflow is enabled. That is to say, the frame stack logic 202 can switchfrom FIFO to LIFO, as described above, and the optical flow tracks logic204 can similarly adapt how its tracks are pruned, as described above.The thresholds Th1 and Th2 can be made configurable. Th1 may be chosen,for example, to be between 0.02 meters to 0.05 meters. Th2 may bechosen, for example, as K×Th1, where K is the sliding window size. If,however, both conditions are false, then the vehicle may be said to bein motion and the SfM system 11 may operate the same as SfM system 10.

The use of the dual thresholds as described can prevent the failure thatmay occur when a number of small translations over a series of framesaccumulatively result in sufficiently large (i.e., superthreshold)camera translation even though the interframe translation between pairsof frames considered individually may be too small to be detected asmotion by a single threshold. Under the described failure condition, theoptical tracker of the SfM system may stop working reliably because overa period of time the accumulated motion may be too large to findreliable tracks.

The below table provides examples of several conditions illustrating thefunctioning of stationary check 206. The examples have a sliding windowsize of 2 frames. In a first example, the magnitude of the differencebetween the camera translation vectors of the first two frames isgreater than a first threshold Th₁ and is also greater than a secondthreshold Th₂. Thus, the camera is considered to be moving, whichdetermination may be communicated to frame stack logic 202 and opticalflow tracks logic 204, as illustrated in FIG. 2B.

In a second example, the magnitude of the difference between a frame 3camera translation vector T₃ and a frame 2 camera translation vector T₂is less than the first threshold Th₁, and the magnitude of thedifference between the frame 3 camera translation vector T₃ and thecamera translation vector of the last moving frame, which in this casehappens to be T₂ from frame 2, is less than the second threshold Th₂.Thus, because at least one of the threshold conditions is met—in thiscase, they both happen to be met—the camera is considered to bestationary, which determination may be communicated to frame stack logic202 and optical flow tracks logic 204, as illustrated in FIG. 2B. Thethird example is similar to the second.

In a fourth example, the magnitude of the difference between a frame 5camera translation vector T₅ and a frame 4 camera translation vector T₄is greater than the first threshold Th₁, but the magnitude of thedifference between the frame 5 camera translation vector T₅ and thecamera translation vector of the last moving frame, which again happensto be T₂ from frame 2, is less than the second threshold Th₂. Thus,because at least one of the threshold conditions is met—in this case,only the second threshold condition is met—the camera is considered tobe stationary, which determination may be communicated to frame stacklogic 202 and optical flow tracks logic 204, as illustrated in FIG. 2B.

The fifth and sixth examples are similar to the first example in thatneither threshold condition is met, and the vehicle and camera aredetermined to be moving. In the sixth example, however, the secondthreshold condition tests the difference in magnitude between frame 7'stranslation vector T₇ and frame 6's translation vector T₆, since frame 6is now considered to be the last moving frame, rather than frame 2.

Sliding window size for triangulation = 2 Threshold Condition 1 < Th₁Threshold condition 2 < Th₂ Result Frame 1 Frame 2 Abs(T₂ − T₁) < Th₁ =FALSE Abs(T₂ − T₁) < Th₂ = FALSE Not stationary Frame 2 Frame 3 Abs(T₃ −T₂) < Th₁ = TRUE Abs(T₃ − T₂) < Th₂ = TRUE Stationary Frame 2 Frame 4Abs(T₄ − T₃) < Th₁ = TRUE Abs(T₄ − T₂) < Th₂ = TRUE Stationary Frame 2Frame 5 Abs(T₅ − T₄) < Th₁ = FALSE Abs(T₅ − T₂) < Th₂ = TRUE StationaryFrame 2 Frame 6 Abs(T₆ − T₅) < Th₁ = FALSE Abs(T₆ − T₂) < Th₂ = FALSENot stationary Frame 6 Frame 7 Abs(T₇ − T₆) < Th₁ = FALSE Abs(T₇ − T₆) <Th₂ = FALSE Not stationary

The present disclosure thus provides three examples that can work inconjunction. First, an adaptive frame stack update logic 202 can convertfrom FIFO to LIFO when a stationary frame is encountered such that thereis sufficient baseline for triangulation between the stationary frameand the reference frame. Second, an optical flow tracks logic 204 canperform matches with respect to a reference frame, even when thestationary camera based frame stack update logic is enabled. Third, astationary check 206 can determine whether a vehicle and camera arestationary or moving using two alternative threshold conditions based onthe translation vector obtained from the pose matrix, and thisdetermination can be used to activate the stationary-camera modes ofeither or both of the adaptive frame stack update logic 202 and/or theoptical flow tracks logic 204.

FIGS. 9-14 illustrate methods that can correspond to the above-describedexamples.

FIG. 9 is a flow chart illustrating a method 900 of controlling avehicle using an SfM system having an adaptive frame stack update logic,such as frame stack update logic 202 shown in FIG. 2B. First, the systemcan acquire 910, from a camera on a vehicle, a sequence of image framesin a frame stack having an update scheme. The camera can correspond tocamera 3 in FIG. 1, and the vehicle can correspond to vehicle 1 inFIG. 1. Second, the system can determine 920 that the vehicle and cameraare stationary. This determination can be made, for example, using thestationary check 206, and can be informed by one or both of an IMUand/or through vision-based methods such as camera pose estimation.

Third, based on determining 920 that the vehicle and camera arestationary, the system can modify 930 the frame stack update scheme fromFIFO to LIFO, as shown and described above with reference to FIGS.6D-6F. Fourth, the system can compute 940 a 3D point cloud based on theimage frames in the frame stack. Fifth, the system can control 950 thevehicle based on the 3D point cloud. This control 950 can be done, forexample, by vehicle controller 50 in FIG. 1. Simple examples of suchcontrol might be applying the brakes or inhibiting acceleration when thepoint cloud indicates that there is an object or another vehicle withina threshold distance or looming at a threshold speed.

FIG. 10 is a flow chart illustrating a method 1000 of controlling avehicle using an SfM system having an optical flow tracks logic, such asoptical flow tracks logic 204 shown in FIG. 2B. First, the system canacquire 1010 a sequence of image frames from a camera on a vehicle. Thecamera can correspond to camera 3 in FIG. 1, and the vehicle cancorrespond to vehicle 1 in FIG. 1. For each frame in the sequence afteran initial frame, the system can compute 1020 a set of tracks based onfeature points in the frame and an earlier frame. For each set oftracks, the system can prune 1030 the set based on the previouslycomputed set of tracks in accordance with a pruning scheme.

The system can determine 1040 that the vehicle and camera arestationary. This determination can be made, for example, using thestationary check 206, and can be informed by one or both of an IMUand/or through vision-based methods such as camera pose estimation.Based on determining 1040 that the vehicle and camera are stationary,the system can modify 1050 the pruning scheme from pruning based on thepreviously computed set of tracks to pruning based on the last set oftracks computed from a frame acquired while the vehicle was moving, asdescribed above with reference to FIGS. 8H and 8I. The system can thencompute 1060 a 3D point cloud based on the sets of tracks and cancontrol 1070 the vehicle based on the 3D point cloud. The computation1060 and control 1070 can be similar to the computation 940 and control950 described above with reference to FIG. 9.

FIG. 11 is a flow chart illustrating a method 1100 of controlling avehicle using an SfM system having a stationary check, such asstationary check 206 shown in FIG. 2B. First, the system can acquire1110 a sequence of image frames from a camera on a vehicle. The cameracan correspond to camera 3 in FIG. 1, and the vehicle can correspond tovehicle 1 in FIG. 1. For each acquired frame, the system can estimatethe camera pose and compute 1120 a translation vector relating thecamera pose to a reference position in the sequence after an initialframe.

Then, the system can make two different comparisons 1130, 1140, whichcomparisons can be done in either order or in parallel. The system cancompare 1130 the magnitude of the difference between the translationvectors corresponding to captured image frames that are consecutive intime with a first threshold. The first threshold can be chosen, forexample, to be between 0.02 meters to 0.05 meters. The system can alsocompare 1140 the magnitude of the difference between the translationvector corresponding to the most recently acquired frame and thetranslation vector corresponding to the last frame acquired while thecamera was moving with a second threshold. The second threshold can bechosen, for example, as the product of the first threshold and a slidingwindow size indicative of the number of frames in a frame stack used tomake the structure from motion computation later at 1160.

Based on both of the comparisons, the system can determine 1150 that thevehicle and camera are stationary and modify the behavior of astructure-from-motion depth determination system. For example, thesystem can modify the behavior of a frame stack logic, such as framestack logic 202 described above with respect to FIG. 2B. For example,the system can alter the frame stack logic from FIFO to LIFO, asdescribed with reference to FIGS. 6D-6F. The system may additionallyalter an optical flow tracks logic, such as optical flow tracks logic204 in FIG. 2B, in the manner described above with regard to FIGS.8H-8I.

More specifically, as described previously, the system can make thedetermination that the vehicle and camera are stationary if either orboth of the following are true: (1) the magnitude of the differencebetween the translation vectors corresponding to captured image framesthat are consecutive in time is less than the first threshold, and (2)the magnitude of the difference between the translation vectorcorresponding to the most recently acquired frame and the translationvector corresponding to the last frame acquired while the camera wasmoving is less than the second threshold.

The system can then determine 1160 depths based on the sequence of imageframes. The depths might be determined, for example, by using SfM tocompute a 3D point cloud from the acquired image frames, which pointcloud is indicative of depths, i.e., distances between the camera (andhence, the vehicle) and other objects. The system can then control 1170the vehicle based on the determined depths, in the ways discussedpreviously, for example.

FIG. 12 is a flow chart illustrating a method 1200 of controlling avehicle using an SfM system having a adaptive frame stack update logic,such as frame stack update logic 202 shown in FIG. 2B. The acquiring1210, determining 1220, and modifying 1230 can be identical to theacquiring 910, determining 920, and modifying 930 in FIG. 9. However,the method 1200 can also determine 1240 that the vehicle and camera aremoving again after having stopped. Based on this determination, thesystem can modify 1250 the frame stack update scheme from LIFO to FIFO,as shown and described above with reference to FIG. 6G. Then, the systemcan compute 1260 a 3D point cloud based on the image frames in the framestack and can control 1270 the vehicle based on the 3D point cloud,similar to the computation 940 and control 950 in FIG. 9.

FIG. 13 is a flow chart illustrating a method 1300 of controlling avehicle using an SfM system having a optical flow tracks logic, such asoptical flow tracks logic 204 shown in FIG. 2B. The acquiring 1310,computing 1320, pruning 1330, determining 1340, and modifying 1350 canbe identical to the acquiring 1010, computing 1020, pruning 1030,determining 1040, and modifying 1040 in FIG. 10. However, the method1300 can also determine 1360 that the vehicle and camera are movingagain after having stopped. Based on determining 1360 that the vehicleand camera are moving, the system can modify 1370 the pruning schemefrom pruning based on the last set of tracks computed from a frameacquired while the vehicle was moving to pruning based on the previouslycomputed set of tracks, as described above with reference to FIG. 8J.The system can then compute 1380 a 3D point cloud based on the sets oftracks and can control 1390 the vehicle based on the 3D point cloud,similar to the computing 1060 and controlling 1070 in FIG. 10.

FIG. 14 is a flow chart illustrating a method 1400 of controlling avehicle using an SfM system having a stationary check, such asstationary check 206 shown in FIG. 2B. The acquiring 1410, estimatingand computing 1420, comparing 1430 and 1440, and determining andmodifying 1450 can be identical to the acquiring 1110, estimating andcomputing 1120, comparing 1130 and 1140, and determining and modifying1150 in FIG. 11. However, the method 1400 can also repeat 1460 thecomparisons 1430 and 1440 using a new most recently acquired frame. Thatis, some time after the comparisons 1430, 1440 are made, after at leastone new frame has been acquired, the system can, in either order or inparallel, compare the magnitude of the difference between thetranslation vectors corresponding to captured image frames that areconsecutive in time with a first threshold, and compare the magnitude ofthe difference between the translation vector corresponding to the newmost recently acquired frame and the translation vector corresponding tothe last frame acquired while the camera was moving with a secondthreshold. The thresholds can be as discussed previously.

Based on both of the repeated comparisons, the system can determine 1470that the vehicle and camera are moving and modify the behavior of astructure-from-motion depth determination system. For example, thesystem can modify the behavior of a frame stack logic, such as framestack logic 202 described above with respect to FIG. 2B. For example,the system can alter the frame stack logic from LIFO to FIFO, asdescribed with reference to FIG. 6G. The system may additionally alteran optical flow tracks logic, such as optical flow tracks logic 204 inFIG. 2B, in the manner described above with regard to FIG. 8J.

More specifically, as described previously, the system can make thedetermination that the vehicle and camera are moving when (and onlywhen) both of the following are true: (1) the magnitude of thedifference between the translation vectors corresponding to capturedimage frames that are consecutive in time is equal to or greater thanthe first threshold, and (2) the magnitude of the difference between thetranslation vector corresponding to the most recently acquired frame andthe translation vector corresponding to the last frame acquired whilethe camera was moving is equal to or greater than the second threshold.

The system can then determine 1480 depths based on the sequence of imageframes, as in 1160 in FIG. 11. The system can then control 1490 thevehicle based on the determined depths, as in 1170 in FIG. 11.

The present systems and methods provide robust SfM that continues toprovide accurate depth information even when the vehicle and camera cometo a halt. The systems and methods of the present disclosure can enablethe usage of monocular cameras even when the vehicle and camera stop,thereby making the monocular camera-based solution more viable due toits robustness and cost-effectiveness. The present systems and methodscan handle changes in a scene such as objects moving into or out of thescene. The present systems and methods are also computationallyefficient, in that, for example, no additional segmentations are needed,or, as another example, the back-up/restore mechanism of optical flowtracks logic 204 does not require repeating optical flow matching forall frames in the frame stack. Furthermore, the described camera poseand thresholding approach to determine when the stationary camera flowhas to be enabled or disabled may yield more accurate results thanapproaches that use only a single threshold condition, for the reasondiscussed previously.

While this disclosure has discussed its methods and systems in terms ofmonocular examples (i.e., involving a single camera), astructure-from-motion system can use multiple cameras and/or multipleprocessing systems to derive depth information about the surroundingscene. For example, multiple outward-facing cameras may be placed aboutthe perimeter of a vehicle so as to acquire 2D information about thesurrounding scene from multiple directions. Such information can then beprocessed by an SfM system, or multiple SfM systems running in parallel,and the resultant 3D data can be merged into a single representation orunderstanding of the surrounding scene. In some examples, multiplecameras may be placed such that front peripheral vision is provided. Inother examples, complete 360-degree view of the surrounding environmentcan be captured and processed, thereby eliminating “blind spots” in thesystem.

What have been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. The term “based on” means based at leastin part on. Additionally, where the disclosure or claims recite “a,”“an,” “a first,” or “another” element, or the equivalent thereof, itshould be interpreted to include one or more than one such element,neither requiring nor excluding two or more such elements.

What is claimed is:
 1. An automotive system comprising: a processor; anda non-transitory computer readable storage medium storing instructionsthat, when executed by the processor, causes the processor to: receive asequence of image frames from a camera; determine whether a vehicleassociated with the sequence of image frames is stationary; in responseto determining that the vehicle is stationary, change an order forstoring the sequence of image frames in a frame stack from a firsttechnique to a second technique that is different from the firsttechnique; and store a first image frame of the sequence of image framesin the frame stack according to the second technique.
 2. The automotivesystem of claim 1, wherein the instructions include further instructionsthat cause the processor to: in response to determining that the vehicleis in motion, change the order for storing the sequence of image framesin the frame stack from the second technique to the first technique; andstore a second image frame of the sequence of image frames in the framestack according to the first technique.
 3. The automotive system ofclaim 1, wherein the instructions include further instructions thatcause the processor to: compute depth values based on the sequence ofimage frames as stored in the frame stack; and control an operation ofthe vehicle according to the depth values.
 4. The automotive system ofclaim 1, wherein the first technique is a first in, first out techniqueand the second technique is a last in, first out technique.
 5. Theautomotive system of claim 1, wherein the instructions that cause theprocessor to determine whether the vehicle is stationary includeinstructions to: estimate a pose of the camera; use the sequence ofimage frames to compute a set of translation vectors relating the poseof the camera to a reference position; determine a first differencebetween a subset of the set of translations vectors corresponding toconsecutive frames of the sequence of image frames; and determinewhether the vehicle is stationary based on the first difference.
 6. Theautomotive system of claim 5, wherein the instructions that cause theprocessor to determine whether the vehicle is stationary include furtherinstructions to: determine a second difference between a first vector ofthe set of translation vectors corresponding to a most recently acquiredframe of the sequence of image frames and a second vector of the set oftranslation vectors corresponding to a most recent frame of the sequenceof image frames that was acquired while the vehicle was in motion; anddetermine whether the vehicle is stationary further based on the seconddifference.
 7. The automotive system of claim 6, wherein theinstructions that cause the processor to determine whether the vehicleis stationary include further instructions to: compare the firstdifference to a first threshold; compare the second difference to asecond threshold; and determine whether the vehicle is stationaryfurther based on the comparison of the first difference to the firstthreshold and the comparison of the second different to the secondthreshold.
 8. The automotive system of claim 7, wherein the secondthreshold is based on the first threshold and a sliding window sizeequal to a number of frames in the frame stack.
 9. The automotive systemof claim 1, wherein the instructions include further instructions thatcause the processor to: prune optical flow tracks generated fromcorresponding feature points in different frames of the sequence ofimage frames; and in response to determining that the vehicle isstationary, change from pruning based on a last computed set of tracksto pruning based on a most recent set of tracks computed from a frameacquired while the vehicle was moving.
 10. The automotive system ofclaim 1, wherein the instructions include further instructions thatcause the processor to: receive, from an inertial measurement unit, aset of inertial measurement data; and determine whether the vehicle isstationary further based on the set of inertial measurement data.
 11. Amethod comprising: receiving a sequence of image frames; determiningfrom the sequence of image frames that a vehicle associated with thesequence of frames is stationary; in response to determining that thevehicle is stationary, changing a technique for storing the sequence ofimage frames in a frame stack from a first technique to a secondtechnique that is different from the first technique; determining fromthe sequence of image frames that the vehicle is in motion; in responseto determining that the vehicle is in motion, changing the technique forstoring the sequence of image frames in the frame stack from the secondtechnique to the first technique; and storing the sequence of imageframes in the frame stack according to at least one of: the firsttechnique and the second technique.
 12. The method of claim 11, whereinthe first technique is a first in, first out technique and the secondtechnique is a last in, first out technique.
 13. The method of claim 11,wherein the determining that the vehicle is stationary includes:detecting an interest point within the sequence of image frames;determining optical flow representing movement of the interest pointbetween frames of the sequence of image frame; and determining that thevehicle is stationary based on the optical flow.
 14. The method of claim11, wherein the determining that the vehicle is stationary includes:performing triangulation using the sequence of image frames to determinea distance between an object and a camera from which the sequence ofimage frames is received; and determining that the vehicle is stationarybased on the triangulated distance.
 15. The method of claim 11, whereinthe determining that the vehicle is stationary includes: determining apose of a camera from which the sequence of image frames is receivedbased on the sequence of image frames; determining translation vectorsfor the sequence of image frames relating the pose to a referenceposition; comparing a first difference between a subset of thetranslation vectors corresponding to consecutive frames of the sequenceof image frames; and determining that the vehicle is stationary based onthe first difference.
 16. The method of claim 15, wherein thedetermining that the vehicle is stationary further includes: determininga second difference between a first translation vector corresponding toa most recently acquired frame of the sequence of image frames and asecond translation vector corresponding to a last frame of the sequenceof image frames acquired while the vehicle was moving; and determiningthat the vehicle is stationary based on the second difference.
 17. Anautomotive system on a vehicle comprising: a camera configured tocapture a sequence of image frames; a structure from motion (SfM) systemcoupled to the camera, the SfM system comprising: a frame stack; aprocessor coupled to the camera and the frame stack to: receive thesequence of image frames from the camera; determine that the vehicle isstationary; select a frame order for storing the sequence of imageframes in the frame stack from between a first ordering technique and asecond ordering technique based on a determination that the vehicle isstationary; store a subset of the sequence of image frames in the framestack according to the selected frame order; compute a depth value basedon the frame stack; and send, to a vehicle controller, the depth value;and the vehicle controller coupled to the SfM system, the vehiclecontroller configured to control operation of the vehicle based on thedepth value.
 18. The automotive system of claim 17, wherein: the firstordering technique is a first in, first out technique; the secondordering technique is a last in, first out technique; and the processoris to select the second ordering technique as the frame order based onthe determination that the vehicle is stationary.
 19. The automotivesystem of claim 17, wherein: the frame order is a first frame order; theprocessor is further to: determine that the vehicle is in motion; selecta second frame order for storing the sequence of image frames in theframe stack from between the first ordering technique and the secondordering technique based on a determination that the vehicle is inmotion; and the second frame order is different from the first frameorder.
 20. The automotive system of claim 17, wherein the processor isfurther to: estimate a pose of the camera; use the sequence of imageframes to compute a set of translation vectors relating the pose of thecamera to a reference position; determine a difference between a subsetof the set of translations vectors corresponding to consecutive framesof the sequence of image frames; and determine that the vehicle isstationary based on the difference.